Copying of data is useful for backup purposes to avoid the loss of data. As discussed in the incorporated '896 USPAP, in data storage systems, such as enterprise storage servers (ESS), for example, the IBM® TotalStorage Enterprise Storage Server™, there may be copy functions, one of which, such as FlashCopy™, that provides point-in-time copy functions. As discussed in the incorporated '896 USPAP, point-in-time copying techniques provide an instantaneous copy or view of what the original data looked like at a specific point in time.
The data may be interdependent such that data of one volume is dependent on data of another volume. However, source data may be subject to updating while the point-in-time data is being copied, and an update to some data that has not yet been copied may thus conflict with interdependent data that has already been copied, and, if the data not yet copied is then copied, the data is inconsistent, and essentially corrupted. In the incorporated '896 USPAP, while performing a copy operation to copy from a source to a target, a write operation is restricted from being performed on the one source, until the copy operations have been performed on the data that is interdependent.
The group of interdependent data that is being copied may be called a consistency group. The write restriction may be called a “FREEZE”, and subsequent writes to the frozen source volume receive a “long busy” condition. When all of the consistency group relationships are established and the interdependent data is copied, a “consistency group created” command is issued to each logical subsystem, causing the “frozen” volumes to be “THAWED”.
In order to keep from an unexpected problem, or a user forgetting to issue a thaw, a timer is started by ESS when the freeze is initiated. If a volume is not thawed within a specified amount of time, it will be automatically thawed, which may lead to an uncertainty whether the group is in fact consistent.